Which means FTB is only getting 1.25GB which really isn't enough. I don't know exactly how much you need, but I would think at least 2GB allocated to it.

If your computer does have more than 2GB though, it means you're likely running 32-bit version of Java which can only use a maximum of 2GB of RAM. If you are running a 64-bit version of Windows then install the 64-bit version of Java and it'll be able to use everything. Then in the launcher you can actually manually click to say how much to use.

Any big 1.6 modpack needs a 64bit OS, due to it requiring more than 1GB of ram. 32bit Java can't manage more than about 1.5GB meaning you can't run java apps that need 2GB.There is only one 1.6.4 FTB pack that works on 32bit computers, which is Lite 2.